In the past, window-cutting systems have involved the use of whipstocks and multiple trips with a starter mill and a window mill coming in behind it to cut out a window. Until recently, there has been no emphasis placed on retrievability of the whipstock in that the older designs and applications presuppose that the whipstock would remain in the wellbore after the window was milled.
More recently, the concept of retrievability of whipstocks has been introduced for a variety of applications. In some instances, the liner which is inserted through the window is cemented all the way back into the main wellbore. Thereafter, milling is required to remove the section of liner extending into the main wellbore. This procedure is illustrated in U.S. Pat. No. 5,301,760. Various attempts in the past have been made to retrieve whipstocks. One such tool is illustrated in U.S. Pat. No. 5,341,873, assigned to Weatherford. These retrieving techniques employed in the past generally required that the structural integrity of the upper end of the whipstock be maintained so that the retrieving tool could get a firm grip on the whipstock to ensure its removal.
Milling techniques have also improved so that a one-trip system can be employed to create the window. U.S. Pat. No. 5,109,924 illustrates a one-trip window-milling system where a started mill is followed by one or more watermelon mills. The assembly is initially retained to the whipstock by a lug and a shear pin.
FIG. 1 illustrates a lug of the type previously employed, with standard multi-trip window-milling systems as well as a one-trip system such as illustrated in Jurgens.
The initial contact wearing surface 10 was previously held at approximately an angle of 2-5.degree. as represented by "a" in FIG. 1. The starter mill 12 had a guide section 14 which was designed to contact the lug 16. In view of the speed of rotation of the starter mill 12 and the small angle "a" employed, the wear patterns on lug 16 were such that it would be quickly ground away before the starter mill 12 could get much of a bite into the casing 18. When this occurred, the mill would be driven away from the casing 18 so that it would retract from an initial window which had just started to open as the mill 12 is further advanced downwardly. If this was allowed to occur, eventually the starting mill 12 ground away the top of the whipstock 20 to a point represented by dashed line 22. This technique was somewhat hit or miss and frequently resulted in severe damage to the top of the whipstock 20. Such damage was generally sufficiently extensive to prevent or at least make extremely difficult any attempt to recover the whipstock 20 from the wellbore. This is because holes conveniently placed near the top of the whipstock for retrieval purposes would be one of the first things ground up if the blades of the starting mill 12 were allowed to progress into contact with the whipstock 20.
The shortcomings of the prior designs were due to the lug design and an effect called "whirl," which is best illustrated in FIG. 2. FIG. 2 schematically illustrates in a plan view a casing 18, along with a starter mill 12, which has a series of blades 24 thereon. The blades are designed to create the cutting action when engaged against the casing 18 due to a clockwise rotation of the starter mill 12, as illustrated by arrow 26. However, since the mill 12 is itself smaller than the opening in which it is disposed, the clockwise rotation imparted to the starter mill 12 as indicated by arrow 26 results in the entire mill 12 rotating in a counterclockwise manner illustrated by arrow 28 within its surroundings. Since the initial surroundings about the starting mill 12 are larger than the O.D. of the mill, the whirl effect creates contact between the blades 24 and the casing 18 such that an undesirable force in the direction of arrow 30 is applied to each of the blades as the starter mill 12 whirls in a counterclockwise direction indicated by arrow 28. In the past, this whirl effect has resulted in severe damage to the starter mill 12 and in many cases to the whipstock 20. The whirl action further exacerbated the wearing away of the lug 16.
While in past designs the objective of beginning a window may have been accomplished, this achievement was at the cost of near complete destruction of the starter mill 12 as well as sufficient damage to the top end of the whipstock 20 to eliminate or at least make difficult subsequent attempts to retrieve it.
One of the many objectives of this invention is to provide guidance and stabilization to the mill or mills through the use of the configuration of the lug to remove the effect of whirl and to spare the whipstock from damage during the process of milling the window in the casing 18. To that end, a sacrificial lug including an initial contact taper and a back-up shoulder has been developed. The taper allows applied weight on the mill during the window milling to more directly orient the mill toward the casing where the window is to be cut. Greater torque control is possible due to the improved guidance of the mill or mills which reduces stall-outs when the mill gets stuck. The lug configuration is directed to the objective of providing a wearing surface rather than a surface that is milled during the creation of the window. Yet another objective is to preserve any retrieving slots or other protrusions used for subsequent retrieval of the whipstock by ensuring that the mill or mills do not destroy such features during the window-milling process.